Document how to run on subdomains

app/scss/main.scss

@@ -432,7 +432,7 @@ a.label-btn {
     .activity-attachment {
         margin: 30px 0 20px 0;
         img, audio, video {
-            max-width: 740px;
+            max-width: calc(min(740px, 100%));
         }
     }
     img.inline-img {

docs/install.md

@@ -191,6 +191,29 @@ http {
 }
 ```
 
+
+## (Advanced) Running on a subdomain
+
+It is possible to run microblogpub on a subdomain (`sub.domain.tld`) while being reachable from the root root domain (`domain.tld`) using the `name@domain.tld` handle.
+
+This requires forwarding/proxying requests from the root domain to the subdomain, for example using NGINX:
+
+```nginx
+location /.well-known/webfinger {
+  add_header Access-Control-Allow-Origin '*';
+  return 301 https://sub.domain.tld$request_uri;
+}
+```
+
+And updating `data/profile.toml` to specify the root domain as the webfinger domain:
+
+```toml
+webfinger_domain = "domain.tld"
+```
+
+Once configured correctly, people will be able to follow you using `name@domain.tld`, while using `sub.domain.tld` for the web interface.
+
+
 ## (Advanced) Running from subpath
 
 It is possible to configure microblogpub to run from subpath.